1. Field of the Invention
The present invention relates to a method and apparatus for generating a composite signal and, more particularly, to a programmable waveform generator operable as an interplex modulator to produce composite, constant-envelope signals.
2. Description of the Related Art
Combining multiple signals on the same radio frequency (RF) carrier is often desirable in both one-way and two-way communications systems, and the importance of signal combining techniques will grow as RF communications systems continue to proliferate and RF spectrum becomes increasingly crowded. Existing methods of signal combining include techniques that generate composite signals whose instantaneous power varies with time (non-constant-envelope signals), such as linear signal combination. Other existing techniques, such as conventional phase shift keyed/phase modulated (PSK/PM) systems, generate constant-envelope composite signals.
Linear methods that generate non-constant-envelope composite signals result in power-inefficient mechanizations, because the power amplifiers that are used for transmission of the composite signals must operate in the linear region. Power amplifiers are much more efficient when operated in the saturated mode. Therefore, constant-envelope signal structures are required if full-power, undistorted transmission is sought.
For example, in a CDMA cellular telephone system, linear superposition of chip-synchronous, orthogonal signals to be transmitted from a base station is a theoretically lossless multiplex if the subsequent transmission chain remains linear. Maintaining linearity requires a linear high power amplifier (HPA). Since any HPA characteristic eventually saturates as its input power increases, such base station transceiver linear amplifiers are typically run at 4–5 dB average power backoff to accommodate peak power needs. In addition, the rather severe spectral containment filtering applied to each user signal before multiplexing creates amplitude fluctuations of 4–5 dB peak-to-average power, requiring additional backoff. Consequently, total backoff can easily be 9 or 10 dB in this particular context.
Thus, linear combination techniques are maximally efficient in the sense that there is no actual signal power loss, but the overall efficiency of such techniques is compromised by the need to operate the amplifier at a significant power back-off to accommodate the instantaneous signal envelope fluctuations. Further, conventional PSK/PM systems have limited power efficiency, because PSK/PM systems include unmodulated carrier and cross modulation terms, which represent wasted power.
An alternative approach to producing greater average power is to achieve a more effective allocation of the loss budget between the multiplexer and the power amplifier. Applied to orthogonal waveforms, non-linear multiplex methods that produce a composite constant-envelope signal permit a greater fraction of the available transmitter power to be used for communication, but at the expense of a multiplexing loss that may be characterized as either cross-talk (induced non-orthogonality or harmonic distortion) or receiver cross-correlation mismatch. This multiplexing loss, however, is typically smaller than the power backoff it replaces, resulting in a favorable trade.
The Global Positioning System (GPS) is another application in which constant-envelope signals would be beneficial. This system includes a constellation of Earth-orbiting satellites that transmit signals useful for determining position. By measuring the time delay in broadcasted signals received from several of these satellites, a receiver can determine its own position using trilateration. Continually evolving GPS system requirements necessitate the simultaneous transmission of multiple signals from each of the GPS satellites, making constant-envelope signals of great interest in developing future GPS signal structures and system architectures.
As military and civilian requirements for GPS change over time, operational modifications will continue to be necessary. Critical signaling parameters, such as chip rates, code types, fixed carrier offset, hopping sequences for hopped carrier offset, and relative power ratios, may require modification throughout the operational life of a satellite. Thus, in addition to having the capability to produce constant-envelope signals, the waveform generator onboard each GPS satellite must be remotely reprogrammable to support generation of a variety of possible future signaling waveforms.
Interplex Modulation is one technique gaining consideration for generating constant-envelope, phase modulated composite signals that offers improved efficiency over standard PSK/PM systems. The interplex modulation technique is described by Siegel et al. in “Communication Satellite Integrity and Navigation Payload on DSCS”, Annual Meeting of the Institute of Navigation, Cambridge, Mass. June 1993, the disclosure of which is incorporated herein by reference in its entirety. Using interplex modulation, three or more signals can be combined to generate a constant-envelope composite signal with minimal combining losses. Again, a constant-envelope composite signal is highly desirable so that a highly-efficient saturated power amplifier can be used.
FIG. 1 is a schematic representation illustrating a typical interplex modulator for combining three signals. Input signals S1, S2 and S3 are digital bitstreams of logical ones and zeros. In FIG. 1, the input signals are shown in “analog” representation, meaning the signals assume the values of −1 and +1, corresponding to the logic values 1 and 0, respectively. Analog multipliers 10 and 12 perform analog multiplications of S1 times S2 and S1 times S3, respectively. Analog gain element 14 places a gain of β1 on the product S1S2, analog gain element 16 places a gain of π/2 on S1, and analog gain element 18 places a gain of β2 on the product S1S3. An analog summer 20 sums the outputs of the analog gain elements and supplies the sum to a linear phase modulator 22. Linear phase modulator 22 also receives a Sin(ωt) carrier signal and modulates the sum signal with the carrier signal to produce the composite constant-envelope output signal v(t) for transmission. The phase modulator has a gain of 1 radian per unit input; therefore, the output from the phase modulator from a unit input has a one radian phase deviation of the Sin(ωt) carrier. Accordingly, the output of the phase modulator is:v(t)=Sin(ωt+S1S2β1+S1π/2+S1S3β2)  (1)
From interplex modulation theory, it is known that the output transmission signal v(t) given by equation (1) can be equivalently expressed as:v(t)=S1 Cos(β1)Cos(β2)Sin(ωt)+S2 Sin(β1)Cos(β2)Cos(ωt)+S3 Cos(β1)Sin(β2)Cos(ωt)−S1S2S3 Sin(β1)Sin(β2)Sin(ωt)  (2)where 0≦β1≦π/2 radians and 0≦β2≦π/2 radians and therefore Sin(β1), Sin(β2), Cos(β1), and Cos(β2)≧0, such that the computed signal attenuations are never negative.
The resulting modulator output signal v(t) has a constant envelope; thus, a saturated amplifier can be used to transmit this signal without backoff. The first three terms in equation (2) correspond to the desired signal terms S1, S2 and S3, respectively. The fourth term is an intermodulation (IM) product, which is an undesired term generated by the modulator. Although the IM product consumes some of the available power, the IM product serves to keep the amplitude of the composite signal envelope constant, which in turn facilitates use of saturated amplifiers.
The conventional interplex modulation scheme shown in FIG. 1 suffers from a variety of limitations. The architecture of conventional waveform generators dictates generating the entire composite signaling waveform as a baseband signal and then up-converting the composite baseband signal to the broadcast radio frequency. While this architecture can be used in certain communication systems, such an approach is not suitable for microwave systems, such as GPS, because the baseband frequency is too low to preclude harmonic and intermodulation interference with the desired microwave output. Moreover, timejitter in required digital-to-analog converters adds phase noise onto the desired output signal. Further, in the up-conversion process, the bandpass filters required for each mixing stage produce ringing at phase transitions that generate amplitude envelope variations, which interfere with the efficiency of the saturated high-power amplifiers required for low-power consumption. A result of this non-constant-envelope is signal distortion that adversely impact Bit Error Rate in CDMA systems and navigation accuracy in GPS applications.
A programmable waveform generator suitable for generating constant-envelope composite signals for a GPS system via interplex modulation techniques is described in U.S. patent application Ser. No. 09/205,510 entitled “Programmable Waveform Generator for a Global Positioning System”, filed Dec. 4, 1998, the disclosure of which is incorporated herein by reference in its entirety. As described therein, the waveform generator individually generates the three signal components and the intermodulation product as binary signals. The four binary signals are then sent to the modulators and used to directly modulate the RF carrier. Specifically, the four signals are respectively fed to four separate BPSK modulators which modulate either the in-phase or quadrature phase component of the RF carrier. The outputs of the modulators are scaled using variable attenuators to achieve the desired relative power ratios among the four signal components. The final RF output signal is formed by summing the outputs of the four variable attenuators.
The approach taken in the system of the aforementioned patent application eliminates a number of limitations of conventional interplex modulators in generating a constant-envelope composite signal. In particular, the modulating signal has a much lower frequency content than the modulated intermediate frequency signal in a conventional interplex modulator, thereby avoiding harmonic interference in the resultant composite signal. The modulating signals from the waveform generator are clocked binary signals that are sent directly to the modulators, thereby eliminating the D/A converter and any associated jitter and phase noise. Further, because no up-conversion of the modulated signal is required, no amplitude variation is introduced by bandpass filters.
However, the particular interplex modulator implementations described in the aforementioned patent application require a significant number of hardware components. These components contribute significantly to the overall cost, weight, size and complexity of the system, all of which are of concern in space satellite and commercial CDMA applications such as wireless telephony. Thus, previous methods of imparting amplitude and phase modulation on an RF carrier are seriously limited by low data rates, low achievable RF frequencies or complex hardware implementations. Accordingly, there remains a need for an affordable, flexible waveform generator capable of generating constant-envelope signals with a minimum of hardware components, which can be remotely reprogrammed in the field to support changing operational requirements.